Upon felling and processing of tree trunks, it has to be possible to correctly determine the dimensions of the resulting logs. A remaining problem upon mechanical measuring in a harvesting assembly during operation is to sufficiently accurately determine the diameter or thickness of the logs, which in turn, together with length information, can be used for the determination of the timber volume of the resulting logs. Known harvesting assemblies usually use two or three measurement points in each diameter measuring, which may be insufficient, particularly in case of oval or otherwise irregular trunk cross-sections.